Reactions of 4-benzoyl-4-methylcyclohexa-2,5-dienone in acids: retro-Fries rearrangements

Abstract

Benzoyl-4-methylcyclohexa-2,5-dienone is known as one of the few relatively stable 4acylcyclohexa-2,5-dienones. In attempts to achieve dienone-phenol rearrangements with acyl migration it has been treated with Lewis acids and with non-nucleophilic acids in non-polar solvents. In each case rapid acyl migration occurs to give 4-methylphenyl benzoate by a retro-Fries rearrangement, along with some cleavage to the phenol. The benzoyl group can be trapped by another phenol, and the mechanism is formulated as a dissociation-recombination process with significant leakage of the 4-methylphenol and benzoyl cation. The reaction is compared with the Fries/retro-Fries equilibrium which has been induced between phenyl esters and 2and 4-acyl phenols. Observations are made on the photochemical Fries rearrangement of 4-methylphenyl acetate, which has been suggested to proceed via 4-acetyl-4-methylcyclohexa-2,5-dienone, and of 4-methyl p henyl benzoate.* Extensive measurements of kinetic isotope effects in the orrho photo-Fries reaction of methyl 4-methoxybenzoate gave no evidence of a concerted, intramolecular process. However, the para rearrangement was not ac~essible.'~ Paper 0/02237C